A wireless power transmission or a wireless energy transfer refers to a technology for wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electric energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electric toothbrushes or electric razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
The wireless power transmission using the resonance, which has been actively studied in these days, can obtain the high power transmission efficiency in a distance of several meters as compared with the magnetic induction scheme. According to the wireless power transmission, a resonant coil generating resonance is included in each of a transmission unit and a receiving unit and the high-efficiency transmission is possible when resonance frequencies of two resonant coils exactly match with each other.